One of about a dozen of the strongest Fraunhofer
lines seen
in the Solar spectrum, the A band at 7600 angstoms is due to telluric
lines of molecular oxygen in the Earth's atmosphere.

A-coefficient

Einstein coefficient, where Aji is the
coefficient of spontaneous emission from upper level j to lower level i.

A-number

Atomic Mass Number: The total number of protons
and neutrons in an atom's
nucleus. For example, Oxygen-16 has a mass number of sixteen,
because it has eight protons and eight neutrons.
[C95]

A Shell Stars

A-type stars in which two different types of line
profiles co-exist.
[JJ95]

A Star

A star of spectral type A with a surface
temperature of about
10,000 K, in whose spectrum the Balmer lines of hydrogen attain their
greatest strength. Helium lines can no longer be seen. Some metallic
lines are present; in late A stars the H and K lines of ionized
calcium appear. A0 stars have a color index of zero. Examples of A
stars are Vega and Sirius.
[H76]

ab Variables

A sub-class of Bailey type RR Lyrae variables, having
asymmetric lioght curves of large amplitude.
[H76]

AB Magnitude System

The AB
magnitude system is defined such that for any bandpass or filter being
considered, the magnitude zero-point corresponds to a flux density of
3631 Jy (1 Jy = 1 Jansky = 10-26 W Hz-1
m-2 =
10-23 erg s-1 Hz-1 cm-2)
[BFM03].

ABT

Abbreviation employed in this book to mean
After the Beginning
of Time, which is here defined as the beginning of the expansion of
the Universe. [F88]

(a) Peculiar stars whose metallic lines are as strong
as those of the
F stars but whose hydrogen lines are so strong as to require that they
be classed with the A stars. They are generally short-period
(<300d) spectroscopic binaries with high atmospheric
turbulence and variable spectra, and are slower rotators than normal A
stars. [H76](b) A-type or F-type objects to which no unique
spectral type can be
assigned. Usually the classifier provides a classification according
to the hydrogen, metallic and calcium lines. Also call metallic-line
stars. [JJ95]

A mathematical group of transformations with the
property that the end
result of a series of transformations does not depend on the order in
which they are performed.
[CD99]

Aberration

(a) Defect in the image formed by a lens, mirror
or optical
system. Spherical aberration results when different rays of light are
brought to more than one focus, producing a blurred image or
coma; chromatic aberration when different wavelengths within a
ray of light are brought to more than one focus, producing an image
distorted by colored fringes. Aberration in lenses can be overcome
by the use of an achromatic lens or a combination of lenses
made of glasses of different refractive indices.
[A84](b) The apparent angular displacement of the observed
position of a
celestial object from its
geometric position, caused by the finite
velocity of light in combination with the motions of the observer and
of the observed object. see Aberration, Planetary[S92]

Aberration

A defect in an optical system such that the image is
not a true picture of the object. For instance, colored fringes may
appear, the image may not be focused, or the shape may show
distortion. Techniques of aberration correction exist; these can,
however, be complex and costly.
Chromatic (color) Aberration is found with a single lens;
mirrors do not suffer from chromatic aberration. Because dispersion
always accompanies refractive deviation, the `red' image will be
farther from the lens than the `blue'. Consequently, the image is
surrounded by colored fringes. Chromatic aberration is corrected by
forming a compound lens, whose elements have different refractive
constants.
Spherical Aberration always occurs with rays that are distant
from the axis and incident on a spherical mirror or lens. It is the
cause of the caustic curve. Spherical aberration is corrected by using
parabolic reflecting and refracting surfaces.
Astigmatism affects rays neither close nor parallel to the
axis. The cone of rays through a lens from an off-axis object does not
focus at a point. Instead, two images in the form of short lines are
formed at different distances from the lens. Between the two the image
appears blurred. Mirrors forming images of off-axis points show a
similar defect. The best method of minimizing astigmatism is to reduce
the aperture with stops, thus allowing light only through the center
of the lens.
Coma is rather similar in cause, effect, and correction to
astigmatism. After refraction by a lens, a cone of rays from an
off-axis object tends to have a tadpole-shaped section because of coma.
Distortion is the result of differences in a lens' magnifying
power between different axes. Reduction of aperture is the normal
solution to both coma and distortion.
[DC99]

The component of stellar aberration
resulting from the essentially uniform and rectilinear motion of the
entire Solar System in space. Secular aberration is usually
disregarded. [S92]

Aberration, Spherical

Always occurs with rays that are distant
from the axis and incident on a spherical mirror or lens. It is the
cause of the caustic curve. Spherical aberration is corrected by using
parabolic reflecting and refracting surfaces.

Aberration, Stellar

(a) Difference in a star's apparent
position in the sky from the apparent position it would have if the
Earth were stationary. Such displacement caused by the Earth's
sidereal motion results in an optical positioning difference
of up to about 20.5 seconds of arc, much greater than any displacement
observed by parallax.
[A84](b) Angular displacement in the apparent direction of
a star, due to
Earth's orbital motion (vE) and the finite speed of light
(c). The displacement = arc tan (vE/c) 20'.49. Thus to an Earth-based
observer a star describes an ellipse on the celestial sphere with a
semimajor axis of 20'.49. (The eccentricity of the ellipse is
zero - i.e., a circle - for a star on the ecliptic pole; for a star on the
ecliptic plane the ellipse degenerates into a straight line.)
[H76](c) The apparent angular displacement of the observed
position of a
celestial body resulting from the motion of the observer. Stellar
aberration is divided into diurnal, annual, and secular
components. [S92]

Aberrations

Effects associated with the performance of optical
components which give rise to imperfect optical images.
[McL97]

Ablation

Erosion of an object (generally a meteorite) by
the friction
generated when it passes through the Earth's atmosphere.
[H76]

(a) A measure of the intrinsic brightness of a star or
galaxy. Absolute magnitude is defined as the apparent magnitude the
star or galaxy would have if it were 32.6 light-years (10 parsecs)
from Earth. The lower an object's absolute magnitude, the
greater its intrinsic brightness. For example, the Sun has an
absolute magnitude of +4.83, while Sirius, whose intrinsic brightness
is greater, has an absolute magnitude of +1.43. A star that is one
absolute magnitude brighter than another (e.g., +4 versus +5) is 2.5
times intrinsically brighter; a star that is 5 absolute magnitudes
brighter is 100 times intrinsically brighter; and a star that is 10
absolute magnitudes brighter is 10,000 times intrinsically
brighter. [C95](b) The absolute magnitude (g) of a Solar-System body such
as an asteroid is defined as the brightness at zero phase angle when
the object is 1 AU from the Sun and 1 AU from the observer.
[H76]

Absolute Space

Newtonian space, hypothesized to define a cosmic
reference frame
independent of its content of matter or energy. The existence of
absolute space, enshrined in aether theory, was denied in relativity.
[F88]

A unit defined in terms of fundamental quantities
(such as length, mass, time, and electric charge).
[DC99]

Absolute Zero

(a) The zero value of thermodynamic temperature; 0
kelvin or -273.15°C.
[DC99] (b) The lowest possible temperature, attained when a
system
is at its minimum possible energy. The Kelvin temperature scale sets its
zero point at absolute zero (-273.15° on the Celsius scale, and
-434.07° on the Fahrenheit scale).
[HH98] (c) The temperature at which thermal
disorder completely
disappears, and which is therefore the ultimate limit of
`coldness'. On the Kelvin (absolute) temperature scale it is by
definition the zero of temperature: on the Celsius scale it lies at
about -273 degrees.
[D89]

Absorptance

Symbol: The ratio of the radiant or luminous
flux absorbed by a body or material to the incident flux. It was
formerly called the absorptivity.[DC99]

Absorption

(a) A process in which a gas is taken up by a
liquid or
solid, or in which a liquid is taken up by a solid. In absorption, the
substance absorbed goes into the bulk of the material. Solids that
absorb gases or liquids often have a porous structure. The absorption
of gases in solids is sometimes called sorption. Compare
adsorption. [DC99] (b) Decrease in the intensity of radiation,
representing energy
converted into excitation or ionization of electrons in the region
through which the radiation travels. As contrasted with monochromatic
scattering (in which reemission occurs in all directions at the same
frequency), the inverse process of emission refers to radiation that
is reemitted in general in all directions and at all
frequencies. [H76]

Absorption Band

see Band Spectrum

Absorption Coefficient

Fraction of the incident radiation absorbed at a
certain wavelength per unit thickness of the absorber. The absorption
coefficient is in general a function of temperature, density, and
chemical composition. ( or k in cm-1)
see Lambert's Law.
[H76]

Absorption Edges

Sudden rises superposed on the smooth decrease of
the curve of the
attenuation coefficient, which cause the curve to have a typical
sawtooth aspect. They generally occur at the limit of spectral
lines. [H76]

Absorption Lines

Dark lines in a spectrum,
produced when light or other
electromagnetic radiation coming from a distant source passes through
a gas cloud or similar object closer to the observer. Like
emission lines, absorption lines betray the chemical composition and
velocity of the material that produces them.
[F88]

Absorption of Radiation

No medium transmits radiation without
some energy loss. This loss of energy is called absorption. The energy
is converted to some other form within the medium. see also
Lambert's Law.
[DC99]

Absorption Spectrum

Dark lines superposed on a continuous spectrum,
caused by the
absorption of light passing through a gas of lower temperature than
the continuum light source.
[H76]

Absorption Trough

Range of wavelengths (around 21 cm) at
which atomic
hydrogen absorbs (or emits) radiation; this is a concept used in the
attempt to detect intergalactic matter.
[A84]

Absorptivity

Symbol:
The ratio of the radiant or luminous
flux absorbed by a body or material to the incident flux. It is now
called the Absorptance.
[DC99]

Abundance

(a) The relative amount of a given element among
others; for example, the abundance of oxygen in the Earth's crust is
approximately 50% by weight.
(b) The amount of a nuclide
(stable or radioactive) relative to other nuclides of the same element
in a given sample. The natural abundance is the abundance of a
nuclide as it occurs naturally. For instance, chlorine has two stable
isotopes of masses 35 and 37. The abundance of 35Cl is
75.5% and that of 37Cl is 24.5%. For some elements the
abundance of a particular nuclide depends on the source.
[DC99] (c) The relative amounts of chemical elements. For
example, hydrogen makes up about 75 of the mass of the Universe, so its
"cosmic abundance" is 75%.
[LB90]

Abundance Ratio

The ratio of the number of atoms of an isotope
to the number of atoms of another isotope of the same element in a
sample. [DC99]

Acausal Initial Conditions

Initial conditions that could not have been
caused by any prior physical process.
[LB90]

Acceleration

(a) The SI unit is the meter per
second per second (m s-2). 1. When considering
motion in one dimension, and in unscientific usage, acceleration means
rate of increase of speed. This is a scalar quantity, which can be
positive or negative. Negative values mean that the speed is
decreasing and may be called deceleration or retardation. 2.
In scientific study of motion in two or three dimensions acceleration
means rate of change of velocity; a = dv / dt. This is a vector
quantity having magnitude (which is always positive) and
direction. Whenever speed changes (increasing or decreasing), or
direction changes, or both speed and direction change, this is an
acceleration.
By Newton's second law the net force F acting on a body of mass
m gives it an acceleration a where F = ma.
[DC99] (b) The rate of increase of velocity with time.
[HH98]

Accelerator

(a) A machine for speeding subatomic particles to
high velocity, then
colliding them with a stationary target or with another beam of
particles moving in the opposite direction. (In the latter instance,
the machine may be called a
collider.) At velocities approaching
that of light the mass of the particles increases dramatically, adding
greatly to the energy released on impact. The resulting explosion
promotes the production of exotic particles, which are analyzed
according to their behavior as they fly away through a particle
detector. [F88] (b) Accelerators are machines that use electric fields to accelerate
electrically
charged particles (electrons, protons, and their antiparticles) to higher
energies. If accelerators are linear, they need to be very long to
achieve the
desired energies, so in some, magnets are used to bend the particles
around and back to the starting point, giving them a little extra energy
each time around. [K98]

Accretion

(a) Collection of material together, generally to
form a single
body. [A84](b) A process by which a star accumulates matter as it moves through a
dense cloud of interstellar gas; or, more generally, whereby matter
surrounding a star flows toward it (as in close binaries).
[H76]

Accretion Disk

A disk of gas that accumulates around a center of
gravitational attraction, such as a white dwarf, neutron star, or
black hole. As the gas spirals in, it becomes hot and emits light or
even X-radiation.
[HH98]

Accumulation Theory

The theory by which planetesimals are assumed to
collide with one another and coalesce, eventually sweeping up enough
material to form the planets.
[Silk90]

(a) Lens (or combination of lenses) that brings
different
wavelenghts within a ray of light to a single focus, thus overcoming
chromatic aberration. It was first successfully made by Joseph von
Fraunhofer. [A84] (b) A compound lens whose elements differ in
refractive constant in order to minimize chromatic aberration. Simple
achromatic doublets are formed by combining two lenses of different
glass. The condition for achromatism is:

1P1 +
2P2 = 0

where 1 and
2 are the
dispersive powers of
the glasses of the lenses, and P1 and
P2 are the powers of the lenses. Achromatic lenses
are corrected for chromatic aberration at two different
wavelengths. see also Apochromatic Lens[DC99]

Achromatic Objective

A lens of two or more components with different
refraction indices
(e.g., crown glass and flint glass), used to correct for chromatic
aberration. [H76]

Actinic Radiation

Radiation that can cause a chemical reaction;
for example, ultraviolet radiation is actinic.
[DC99]

Actinium

A soft silvery-white radioactive metallic element that
is the first member of the actinoid series. It occurs in minute
quantities in uranium ores. It can be produced by neutron bombardment
of radium and is used as a source of alpha particles. The metal glows
in the dark.
Symbol: Ac; m.p. 1050±50°C; b.p. 3200±300°C;
r.d. 10.06 (20°C); p.n. 89; most stable isotope 227Ac
(half-life 21.77 years). [DC99]

Action

A quantity related to the momentum and position
of a body or
system of particles. The Principle of Least Action asserts that the
integral, or sum of this action, taken over a particular path must be
a minimum. This Principle of Least Action can be used instead of
Newton's Laws to determine the motion of a system. [P88]

Action-at-a-Distance

A description of a force, such as Newton's law
of gravity, in which two separated bodies are said to directly exert
forces on each other. In the modern description, the bodies produce a
gravitational field, which in turn exerts forces on the two bodies. see
Gravitational Field[G97]

Active Galactic Nucleus

AGN -- An unusually bright galactic nucleus
whose light is not due to starlight. [HH98]

Active Galaxy

Any galaxy which is emitting large quantities of
non-thermal radiation. [C97](b) Active galactic nuclei are
very luminous
(1043-1046 ergs s-1). Their energy
output is in two forms: nonthermal continuum and thermal emission
line. [H76]

Active Optics

Controlling the shape of a telescope mirror at a
relatively slow rate. [McL97]

Active Sun

The Sun during its 11-year cycle of activity when
spots, flares,
prominences, and variations in radiofrequency radiation are at a
maximum. [H76]

Activity

Symbol: A For a radioactive substance, the
average number of atoms disintegrating per unit time. [DC99]

Acuity, Visual

The ability of the eye to see separately two
points close to each other. It is a measure of the resolving power of
the eye's optical system and depends on the density of cells in the
retina. The maximum acuity of the normal human eye is around 0.5
minutes of arc - points separated by this angle at the eye should be
seen as separate. see Resolution[DC99]

Adaptive Optics

Compensating for atmospheric distortions in a
wavefront by high-speed changes in the shape of a small, thin
mirror. [McL97]

ADC

Analog-to-Digital Converter -- An electronic
circuit which takes
an input voltage in a given range (typically 0-10 volts) and provides a
corresponding digital output by setting output lines (bits) high or low.
A 16-bit ADC has 16 output lines. [McL97]

ADF

Astrophysical Data Facility, located at the
NASA Goddard Space
Flight Center (GSFC), is responsible for designing,
developing, and operating data systems that support the
processing, management, archiving and distribution of
NASA
mission data. The
ADF
serves three broadly-defined astrophysics
disciplines: high-energy astrophysics, UV/optical astrophysics,
and infrared/submillimeter/radio astrophysics. The ADF
collaborates with the GSFC Laboratory for High Energy
Astrophysics (LHEA) and the Laboratory for Astronomy and Solar
Physics (LASP) in managing data for specific missions. The ADF
staff also support the astrophysics community's access to
multi-mission and multi-spectral data archives in the National
Space Science Data Center (NSSDC).

Adhesion

A force of attraction between atoms or molecules of
different substances. For example, adhesion between water molecules
and glass creates a meniscus. [DC99]

Adiabatic Change

A change taking place in a system that has
perfect thermal insulation, so that heat cannot enter or leave the
system and energy can only be transferred by work. In practice, a
close approximation to an adiabatic change can be achieved by the
process being too rapid for significant heat transfer, or by the large
scale of the system (e.g. a large volume of air in the atmosphere).
In an adiabatic expansion of a gas, mechanical work is done by the gas
as its volume increases and the gas temperature falls. For an ideal gas
undergoing a reversible adiabatic change it can be shown that

pV =
K1Tp1 - =
K2
and T V -
1 = K3

where K1, K2, and
K3 are constants and
is the ratio
of the principal specific heat capacities. Compare isothermal
change. [DC99]

Adiabatic Demagnetization

A method of producing temperatures
close to absolute zero. A sample of a paramagnetic salt is cooled in
liquid helium in a strong magnetizing field. The sample is then
thermally isolated by pumping away the helium, and the magnetic field
is removed. The sample demagnetizes itself at the expense of its
internal energy so that the temperature falls. Temperatures of the
order of a millikelvin can be obtained. [DC99]

Adiabatic Index ()

The ratio of the fractional change in pressure to
the fractional
change in density as an element of fluid expands (or contracts)
without exchange of heat with its surroundings. [H76]

Adiabatic Fluctuations

Fluctuations in both the matter and radiation
density, as though a volume of the Universe were slightly squeezed but
allowing no radiation to escape. Prior to the Decoupling Era,
adiabatic fluctuations behaved like waves, on scales smaller than the
horizon size. After decoupling, gravitational instability sets in on
scales above about 1013 M, smaller adiabatic
fluctuations having been damped at earlier eras. [Silk90]

Adsorption

A process in which a layer of atoms or molecules of
one substance forms on the surface of a solid or liquid. All solid
surfaces take up layers of gas from the surrounding atmosphere. The
adsorbed layer may be held by chemical bonds (chemisorption) or
by weaker van der Waals' forces (physisorption). Compare
absorption. [DC99]

The slow rotation of the major axis of a planet's
orbit in the
same direction as the revolution of the planet itself, due to
gravitational interactions with other planets and/or other effects
(such as those due to general relativity). [H76]

Advection

The transfer of matter such as water vapor or heat
through the atmosphere as a result of horizontal movement of an air
mass. [DC99]

A stony meteorite, composed primarily of
silicates. About 93
percent of all known falls are aerolites. They include the
carbonaceous chondrites, other chondrites, and achondrites. (lit. "air
stone.") [H76]

After-Image

An image seen after the eye's retina has been
exposed for a time to an intense or stationary light source. It may be
negative or positive, or appear in complementary colors. [DC99]

Aether

(1) In Aristotelian physics, the fifth
element, of which the stars and planets are made. (2) In
Classical physics, an invisible
medium that was thought to suffuse all space. [F88]

Age of the Universe

The time elapsed since the singularity predicted
by the Big Bang theory, estimated to be around 13 billion years.
[BFM02]

AIPS

Astronomical Image Processing System --
National Radio Astronomy Observatory

Airglow

Light in the nighttime sky caused by the
collision of atoms and
molecules (primarily oxygen, OH, and Ne) in Earth's geocorona with
charged particles and X-rays from the Sun or outer space. The airglow
varies with time of night, latitude, and season. It is a minimum at
zenith and maximum about 10° above the horizon. (also called
nightglow)[H76]

Airy Diffraction Disk

The central spot in the diffraction pattern of
the image of a star at the focus of a telescope. Named for Sir George
Airy (1801-1892), seventh Astronomer Royal.
[McL97]

Albedo

(a) The ratio of the amount of light reflected from a
surface to the amount of incident light.
[DC99] (b) Ratio of the total flux reflected in all
directions to the total
incident flux. see Bond Albedo; Geometric Albedo[H76]

Alchemy

Art of bringing parts of the Universe to the
perfect state toward
which they were thought to aspire - e.g., gold for metals, immortality
for human beings.
[F88]

( Tau) -- (a) The brightest star in the constellation Taurus,
Aldebaran
is an orange K-type giant that lies 60 light-years away. [C95](b) A K5 III subgiant (a foreground star in the
Hyades) about 21
parsecs distant. It has a faint M2 V companion. It is now known to be
slowly and irregularly variable. [H76]

-- Theory

Explanation of the big-bang theory in terms of
nuclear physics, proposed by Ralph Alpher, Hans Bethe and George
Gamow in 1948; it was later slightly corrected by Chushiro
Hayashi. [A84]

Alfvén Number

A dimensionless number characterizing steady fluid
flow past an obstacle in
a uniform magnetic field parallel to the direction of flow. It has a
partial analogy to the Mach number. The Alfven number is given by
vl(
µ)1/2B-1/2 where v is the
velocity of flow, l is length of obstacle,
is density, µ is
permeability and B is magnetic flux density. It is named after
H. 0. G. Alfven (1908- ), the Swedish astrophysicist and Nobel Prize
winner, who introduced the term magnetohydrodynamics. [JM92]

Alfvén-Klein Cosmology

A cosmological model in which the early
Universe is depicted as a giant collapsing spherical cloud of matter
and antimatter. When a critical density is reached, the matter and
antimatter begin to annihilate, the resulting release of radiation and
energy causing the Universe to expand. There are many difficulties with
this model of the expanding Universe, which is largely discredited on
observational grounds. [Silk90]

Alfvén Speed

The speed at which hydromagnetic waves are
propagated along a
magnetic field: (VA) = B / (4 )1/2. [H76]

Alfvén Waves

Waves moving perpendicularly through a magnetic
field. They are
caused by the oscillation of magnetic lines of force by the motions of
the fluid element around its equilibrium position, which in turn is
caused by the interactions between density fluctuations and magnetic
variations. [H76]

Algol

( Per) (a) The most famous eclipsing binary, Algol was
probably the first
variable star discovered. It lies in the constellation Perseus and
consists of two stars that orbit each other every 2.87 days. When one
star passes in front of the other, the light of the system dims. [C95](b) An eclipsing system of at least three components
(B8 V, K0, Am),
about 25 pc distant. Period of components A and B is about 68.8 hours;
period of components A, B, and C is about 1.9 years. Long term
observations also indicate a massive, unseen fourth component with a
period of about 190 years. Algol is also an erratic radio source of
about 0.5 AU diameter. [H76]

Aliasing

In a discrete Fourier transform, the overlapping
of replicas of
the basic transform, usually due to undersampling. [H76]

Allotropy

The existence of a solid substance in different
physical forms. Tin, for example, has metallic and non-metallic
crystalline forms. Carbon has two crystalline allotropes: diamond and
graphite. [DC99]

Almagest

Arabic title for Ptolemy of Alexandria's
Syntaxis, the
writings in which he combined his own astronomical researches with
those of others. Although much of the work is inaccurate even in
premise, until Nicolaus Copernicus published his results fourteen
centuries later the Almagest remained the standard reference
source in Europe. [A84]

-Particle

The nucleus of a 4He atom, consisting
of two protons and
two neutrons. Mass of -Particle 4.00260 amu. [H76]

A hypothetical process of nucleosynthesis (now
considered
obsolete terminology), which consisted of redistributing -particles in the region
from 20Ne to 56Fe (and perhaps slightly higher).
The -process has been
replaced by explosive and nonexplosive C, O, and Si burning occurring
in rapidly evolving or even explosive stages of stellar evolution
which at higher temperatures and densities becomes the
e-process. [H76]

Alpha Centauri

(a) Bright binary star in which both components
contribute
to a magnitude of -0.27: it is also the nearest of the bright stars
(at a distance of 4.3 light years). [A84](b) The nearest star system to the Sun and the third
brightest star
in the night sky. Unfortunately, Alpha Centauri is so far south that
it is visible only from latitudes below 25 degrees north. The system
consists of three stars: Alpha Centauri A, the brightest, which is a
yellow G-type main-sequence star like the Sun; Alpha Centauri B, the
second brightest, which is an orange dwarf; and Alpha Centauri C, by
far the faintest, which is a red dwarf. Alpha Centauri A and B both
lie 4.35 light-years from Earth and orbit each other every 80 years;
Alpha Centauri C lies far from its mates and 4.25 light-years from
Earth. Because it is closer to Earth than are A and B, Alpha Centauri
C is usually called Proxima Centauri. [C95]

Alpha Decay

(a) A type of radioactive decay in which the unstable
nucleus emits a helium nucleus. The resulting nuclide has a mass
number decreased by 4 and a proton number decreased by 2. An example
is:

88226 Ra
86222 Rn + 24 He

The particles emitted in alpha decay are alpha
particles. Streams of alpha particles are alpha rays or
alpha radiation. They penetrate a few centimeters of air at STP
or a metal foil of mass/area a few milligram/cm2. see
also Beta Decay[DC99] (b) The disintegration of an atomic nucleus, in
which the
final products are an alpha particle and a nucleus with two fewer
protons and two fewer neutrons than the original. [G97] (c) Spontaneous emission by a heavier element (such
as uranium) of
positively charged helium nuclei - alpha particles - comprising 2
protons and 2 neutrons. The result of this radioactive decay is that
the original element is very gradually converted into another element,
with a decreased atomic number and mass. Alpha particle emission may
be simultaneous with beta particle decay. [A84]

Alpha Particles

Particles first discovered in radioactive
decay, and later
identified as helium nuclei (two protons and two neutrons bound
together). [CD99]

(a) Comprising a means of measuring or precisely
locating in
coordinates the position of objects at any altitude or azimuth. The
term is now used mainly to describe a type of mounting for a
telescope. (also Alt-Az)[A84](b) A form of mounting similar to that
of a radar which allows the telescope tube to be moved horizontally (by
rotation in azimuth or compass direction) and vertically (by rotation in
altitude or elevation). To follow a star the telescope must be adjusted
simultaneously in both axes. [McL97]

Altitude

(a) Angular distance above the horizon. [A84] (b) The angular distance of a celestial body above
or below the
horizon, measured along the great circle passing through the body and
the zenith. Altitude is 90 deg. minus
zenith distance. [S92]

Aluminum

A soft moderately reactive metal. Aluminum has the
electronic structure of neon plus three additional outer
electrons. There are numerous minerals of aluminum; it is the most
common metallic element in the Earth's crust (8.1% by weight) and the
third in order of abundance.
Symbol: Al; m.p. 660.37°C; b.p. 2470°C; r.d. 2.698
(20°C); p.n. 13; r.a.m. 26.981539. [DC99]

A unit of molar volume at 0° C and a
pressure of 1.0 atmosphere.
This unit varies slightly from one gas to another, but in general it
corresponds to 2.24 × 104 cm3. Also, a unit
of density
equal to 0.0446 gram mole per liter at 1 atm pressure. [H76]

A highly toxic radioactive silvery element of the
actinoid series of metals. A transuranic element, it is not found
naturally on Earth but is synthesized from plutonium. 241Am
has been used in gamma-ray radiography.
Symbol: Am; m.p. 1172°C; b.p. 2607°C; r.d. 13.67
(20°C); p.n. 95; most stable isotope 243Am (half-life
7.37 × 103 years). [DC99]

Amorphous

Denoting a solid that has no crystalline structure;
i.e. there is no long-range ordering of atoms. Many substances that
appear to be amorphous are in fact composed of many tiny
crystals. Soot and glass are examples of truly amorphous materials. [DC99]

Ampere

The SI unit of electric current. "The ampere is
that constant
current which, if maintained in two straight parallel conductors of
infinite length, of negligible circular cross-section, and placed 1
meter apart in vacuum, would produce between these conductors a force
equal to 2 × 10-7 newton per meter of length" (CIPM
[1946], Resolution 2, approved by the 9th CGPM 1948). A current of 1 A
is equivalent to the passage along the filament of a light bulb of
about 6 × 1018 electronic charges per second. [H76]

Amplifier

A device that increases an electrical signal applied
to it as an input. If the input is an alternating voltage, the output
voltage has a similar waveform with an increased amplitude.
The ratio of the output signal to the input signal (called the
gain), will usually vary with the signal frequency. Amplifiers
are usually designed to give a particular current, voltage, or power
gain over the required frequency range. Some circuits containing a
number of amplifying stages can cope with frequencies from 0 hertz
(steady direct current) to radiofrequencies. In modern solid-state
electronics, all of the amplifier circuit components, including many
individual amplifying stages, are manufactured in a single integrated
circuit. [DC99]

Amplitude

(a) The maximum value of a varying quantity from
its mean
or base value. In the case of a simple harmonic motion - a wave or
vibration - it is half the maximum peak-to-peak value. [DC99] (b) The maximum height of a wave peak or the maximum
depth of a wave trough. [G99](c) Maximum displacement from the equilibrium
position. [H76](d) Total range of light variation when used in the context of
variable stars.

Amplitude Modulation

(AM) A type of modulation in which the
amplitude of a carrier wave is modulated by an imposed signal, usually
at audio frequency.
In this way communication of a signal is made between
two distant
points using a radio transmission as carrier. When the carrier wave is
received the audio component is extracted by the process of
demodulation, and the original sound may be reproduced. [DC99]

Analyzer

A device for determining the plane of
polarization of plane-polarized radiation. Maximum intensity is
transmitted if the plane is parallel with the analyzer's direction of
polarization; the intensity is a minimum (theoretically zero) if the
two are perpendicular. For visible radiation, analyzers are usually
Polaroid sheets or Nicol prisms. [DC99]

Anamorphic Magnification

The difference in magnification along the
spectrum and perpendicular to the spectrum in a spectrograph. [McL97]

Anastigmastic Lens

A lens designed so as to minimize its
astigmatic aberration. Anastigmatic lenses have different curvatures
in different directions; the surface of an anastigmatic lens is part
of a toroid. [DC99]

Andromeda

A constellation near Perseus and
Pegasus.

Andromeda Galaxy (M31)

(a) Major spiral galaxy, 2.2 million
light-years from
Earth. Gravitationally bound to the
Milky Way galaxy with which it
shares membership in the Local Group,
it is currently approaching us, rather than receding as is the case
for most galaxies. [F88](b) A spiral galaxy (Sb in Hubble's classification; kS5 in Morgan's
classification) in the Local Group, about 650-700 kpc distant
(MV = -21), visible to the naked eye as a fuzzy
patch in the constellation of Andromeda. Total mass about 3.1 x
1011 M ; i = 77°, heliocentric velocity - 180 km
s-1. Its nucleus exhibits noncircular gas motions. It is
similar to but slightly larger than our Galaxy. [H76](c) The largest galaxy in the Local Group. also known as the Great
Spiral and M31. It is about one and a half times the size of our own
galaxy, and contains at least 300 globular clusters. Two smaller,
elliptical galaxies (M32 and NGC 205) lie close to it. [A84]

Andromeda I, II, III

Three dwarf spheroidal galaxies, in the Andromeda
subgroup of the
Local Group, discovered by van den Bergh in 1972. They are among the
intrinsically faintest members of the Local Group. [H76]

(a) A unit that measures the wavelength of light and
equals 0.00000001 of a
centimeter. Blue light has a wavelength of about 4400 angstroms,
yellow light 5500 angstroms, and red light 6500 angstroms. [C95](b) A unit of length equal to 10-10 meters;
symbol Å. Named for
Swedish physicist Anders Jonas Angstrom (1814-1874). [McL97] (c) A unit of length equal to 10-10 m. It
is used in atomic and molecular measurements
and for the wavelength of electromagnetic radiation in the visible, near
infra-red and near ultraviolet regions of the spectrum. The definition
of the
unit is based on the red emission line of the cadmium spectrum which has an
internationally agreed wavelength of 6438.4696 Å in dry air at
standard atmospheric pressure at a temperature of 15°C and
containing 0.03% carbon dioxide by volume.

The unit was introduced by the International Union for Solar Research in
1907[19]. It was named after A. J. Ångström (1814-1874), the
Scandinavian
scientist who used units of 10-10 m to describe wavelengths
in his classical
map of the Solar spectrum made in 1868. The ångström was not
confirmed as
a unit of length by the International Congress of Weights and Measures
until 1927. For over half a century the ångström was equal to
1.0000002 × 10-10 m
but when the metre was defined in terms of the wavelength of krypton in
1960[20] the ångström became equal to 10-10 m
exactly. The ångström is sometimes called a tenth metre.

X-ray wavelengths are often given ångström stars
(Å*). This unit was
devised by J. A. Bearden[21] in 1965 and is based on the wavelength
of the K1 line of
tungsten which he took to be 0.2090100 Å*, where
Å* = 1.00001481 Å = 1.00001481 × 10-10 m. [JM92]

Symbol:
The rotational displacement of
an object about an axis. If the object (or a point on it) moves from
point P1 to point P2 in a plane perpendicular to
the axis, is the angle
P1OP2, where O is the
point at which the perpendicular plane meets the axis. [DC99]

Angular Dispersion

The rate of change of angle (due to refraction or
diffraction) with wavelength of the emergent beam in a
spectrograph. [McL97]

Angular Frequency

(Pulsatance) Symbol:
The number of
complete rotations per unit time. A simple harmonic motion of
frequency f can be represented by a point moving in a circular
path at constant speed. The foot of a perpendicular from the point to
a diameter of the circle moves backward and forward along the diameter
with simple harmonic motion. The angular frequency of this motion is
2f, where f is the
frequency. The unit is the hertz. [DC99]

Angular Momentum

(a) The angular momentum of a system about a
specified origin is the
sum over all the particles in the system (or an integral over the
different elements of the system if it is continuous) of the vector
products of the radius vector joining each particle to the origin and
the momentum of the particle. For a closed system it is conserved by
virtue of the isotropy of space. [H76](b) The product of mass and angular velocity for an
object in
rotation; similar to linear momentum. In
quantum mechanics, angular momentum
is quantized, i.e., is measured in indivisible units equivalent to
Planck's constant divided by 2
pi. This corresponds classically to
only certain frequencies of rotation being allowed.[F88] (c) A property of rotary motion analogous to the
familiar concept of momentum in linear motion. [D89]

Angular Size

The angle subtended by an object on the sky. For
example, the angular size of the moon is 30 arcminutes. [HH98]

Anharmonic Oscillator

A system whose vibration, while still
periodic, cannot be described in terms of simple harmonic motions
(i.e. sinusoidal motions). In such cases, the period of oscillation is
not independent of the amplitude. [DC99]

Anisotropic Models

see Mixmaster Model

Anisotropic Superfluid

A system of fermions in
which Cooper pairs form in a state of finite relative orbital motion
and possibly finite total spin. [D89]

Anisotropy

(a) A medium is anisotropic if a certain physical
quantity differs in value in. different directions. Most crystals are
anisotropic electrically; important polarization properties result
from differences in transmission of electromagnetic radiation in
different directions. [DC99] (b) The condition in which the Universe appears
different in different directions. [LB90](c) The characteristic of being dependent upon
direction. (Light
coming with equal intensity from all directions is
isotropic; a spotlight's beam is
anisotropic.) The
cosmic background radiation is generally
isotropic - i.e., its intensity is the same in all parts of the sky -
but small anisotropies have been detected which are thought to reflect
the earth's proper motion relative to the framework of the Universe
as a whole. [F88]

Annihilation

A reaction between a particle and its
antiparticle; for example, between an electron and a positron. The
energy released is equal to the sum of the rest energies of the
particles and their kinetic energies. In order that momentum be
conserved two photons are formed, moving away in opposite
directions. This radiation (annihilation radiation) is in the
gamma-ray region of the electromagnetic spectrum. The quantum energy
is about 0.51 MeV.
Annihilation also can occur between a nucleon and its
antiparticle. In this case mesons are produced.
see Pair Annihilation[DC99]

Annual Variation

The direction and strength of the Earth's
magnetic field at any point changes with time. This must be allowed
for by navigators. One such change is a variation with a period of a
year, but there are others. The amplitude of the annual variation is
greatest during maximum sun-spot activity. [DC99]

Annular Eclipse

An eclipse of the Sun in which the Moon is too
far from Earth to
block out the Sun completely, so that a ring of sunlight appears
around the Moon. [H76]

Anomalistic Month

The interval (27.555 days) between two successive
perigee passages
of the Moon.[H76]

Anomalistic Year

The interval (365.2596 ephemeris days) between
two successive
perihelion passages of Earth. [H76]

Anomalous Dispersion

The refractive index of a transparent
medium normally increases as the wavelength is reduced. There is then
a range of wavelengths (usually in the ultraviolet) in which the
radiation is absorbed fairly strongly. Such little radiation as is
transmitted in this region shows anomalous dispersion, that is the
refractive index decreases as the wavelength is
reduced. see Dispersion[DC99]

Anomalous Expansion

An increase in volume resulting from a
decreased temperature. Most liquids increase in volume as their
temperature rises. The density of the liquid falls with increased
temperature. Water, however, shows anomalous behavior. Between 0 and
4°C the density increases with increasing temperature. [DC99]

Anomalous Zeeman Effect

Splitting of spectral lines into several
components, in contrast
to the normal Zeeman effect which results in only two distinct
components. The anomalous Zeeman effect is due to the fact that the
electrons in the magnetic field have opposite directions of
spin. [H76]

Anomaly

An angular value used to describe the position of
one member of a
binary system with respect to the other. The true anomaly of a star is
the angular distance (as measured from the central body and in the
direction of the star's motion) between periastron and the observed
position of the star. The mean anomaly is the angular distance
(measured in the same manner) between periastron and a fictitious body
in the direction of the star, which is moving in a circular orbit with
a period equal to that of the star. [H76]

Ansae

(a) The "handles", or extremities, of Saturn's
rings as viewed from
Earth
(b) The extremities of a lenticular galaxy. [H76]

The direction in the sky (in Columba) away from
which the Sun
seems to be moving (at a speed of 19.4 km s-1) relative to
general field stars in the Galaxy. [H76]

Antares

( Sco) (a) A red supergiant star in the constellation
Scorpius. Antares is
the brightest star in Scorpius and lies about 500 light-years from
Earth, on the inner edge of the Orion spiral arm. [C95](b) A red M1 Ib supergiant, about 125 parsecs
distant. It has a B3 V
companion, which is a radio source. [H76]

Antenna

The part of a radio telescope responsible for
detecting an electromagnetic wave. (or Aerial) [McL97]

Antenna Gain

A measure of the directivity of a radio
telescope. It is the ratio
of the amount of power received in the direction the dish is pointing
to the smaller amount of power from other directions in the
sidelobes. [H76]

Antenna Temperature

A term used to describe the strength of a signal
received from a
radio source. It is the convolution of the true brightness
distribution and the effective area of the antenna. [H76]

Antennae

A famous pair of interacting galaxies in the
constellation
Corvus. Each galaxy's tidal force has drawn out a long tail of stars
from the other. The Antennae are also known as NGC 4038 and NGC
4039. [C95]

Anthropic Principle

(a) The doctrine that the value of certain
fundamental constants of
nature can be explained by demonstrating that, were they otherwise,
the Universe could not support life and therefore would contain nobody
capable of worrying about why they are as they are. Were the
strong nuclear force slightly
different in strength, for instance, the stars
could not shine and life as we know it would be impossible. [F88](b) The weak form of the anthropic principle states that life can
exist only during a brief period of the history of our Universe.
The strong form of the principle states that out of all possible
values for the fundamental constants of nature and the
initial conditions of the Universe, only a small fraction could allow
life to form at all, at anytime. see Boundary Conditions;
Fundamental Constants of Nature[LB90]

The direction of the sky (in Auriga) opposite to
that toward the
center of the Milky Way. [H76]

Anti-Coincidence Counter

A particle counter in which the circuit has been
designed so as
not to register the passage of an ionizing particle through more than
one counting tube. [H76]

Anti-Ferromagnet

a solid in which the spins of neighboring atoms are
oppositely aligned. The lattice is composed of two equivalent
sublattices, and on each sublattice the spins are magnetized, as in a
ferromagnet, but the directions of the magnetisations are opposed so
that there is no net magnetisation. [D89]

Anti-Ferromagnetism

A kind of magnetism found in many solids at
low temperatures. The molecular magnets form two arrays, aligned
antiparallel. At the lowest temperatures there are equal numbers with
equal magnetic moments in opposite directions, giving zero resultant
magnetization. As the temperature is raised, the susceptibility
increases up to the Néel temperature above which the
substance is paramagnetic. [DC99]

Anti-Matter

(a) For every variety of particle there exists an
antiparticle with opposite properties such as sign of electrical
charge. When a particle and its antiparticle meet they can mutually
annihilate and produce energy. Thus, antiquark, antiproton, etc. [D89] (b) Atomic particles that have the same mass as, but
opposite
charge and orbital direction to, an ordinary particle. Thus, instead
of negatively charged electrons, atoms of antimatter have positrons. A
quantity of antimatter coming into contact with matter would "cancel
out" - annihilate, with total conversion of mass to energy - an exact
proportion of matter corresponding to the original quantity of
antimatter, provided that the elements in the matter also corresponded
with the "elements" in the antimatter, i.e., that the atoms were
equivalent but opposite. [A84](c) Matter made of particles with
identical mass and
spin as those of ordinary matter,
but with opposite charge. Antimatter has been produced
experimentally, but little of it is found in nature. Why this should
be so is one of the questions that must be answered by any adequate
theory of the early Universe. [F88]

Antimony

A metalloid element existing in three allotropic
forms; the most stable is a brittle silvery metal. It is used in
alloys - small amounts of antimony can harden other metals. It is also
used in semiconductor devices.
Symbol: Sb; m.p. 630.74°C; b.p. 1635°C; r.d. 6.691; p.n. 51;
r.a.m. 112.74. [DC99]

The antiparticle of a neutron. A neutron and
antineutron
both have the same mass and zero electric charge, but can be
differentiated by their interactions: a neutron and an antineutron can
annihilate into gamma rays, while two neutrons cannot. [G97]

Anti-Particle

(a) An elementary particle of opposite charge but
otherwise
identical to its partner. Most of the observable Universe consists of
particles and matter, as opposed to antiparticles and
antimatter. [Silk90](b) For every known type of particle, there exists an
antiparticle with exactly the same mass, but with the opposite
electric charge. When a particle and its antiparticle come together,
they can always annihilate to form gamma rays. The antiparticle of an
electrically neutral particle is sometimes the same as the original
particle (e.g., photons) and sometimes it is distinct (e.g.,
neutrons). [G97](c) Particles predicted by combining the theories of
special relativity
and quantum mechanics. For each particle, there must exist an
antiparticle with the opposite charge, magnetic moment and other
internal quantum numbers (e.g., lepton number, baryon number,
strangeness, charm, etc.), but with the same mass, spin and
lifetime. Note that certain neutral particles (such as the photon and
0) are their own
antiparticles. [CD99] (d) A particle of the same mass and spin, but opposite
charge (and other properties) to its corresponding particle. For
example, a proton and antiproton both have mass 1836 times that of an
electron and spin ½ unit, but the charge on the proton is + 1
unit, while that on the antiproton is -1 unit. For unstable particles,
such as an isolated neutron, the particle and antiparticle have the
same half-life. For uncharged particles the antiparticle is indicated
by a bar above the symbol, such as
for the antineutron. For
charged
particles the distinction is indicated by the sign, for example, e+ is
the positron, the antiparticle of an electron. Antiparticles of
fermions are subject to a conservation law according to which new
particles can only be created in particle-antiparticle pairs, while
particles can be destroyed only by annihilation with their
antiparticles. This rule does not apply to bosons. see also
Fermion[DC99] (e) Every particle has an associated antiparticle, another
particle with the same mass but all charges opposite. If a particle has
no charges, such as the
photon, it is its own antiparticle. Often the antiparticle is denoted
by writing
a bar over the particle's name; hence, for example, the electron
antiparticle (also called the positron) is denoted
.

Anti-Proton

The antiparticle of a proton, identical in
mass and spin
but of opposite (negative) charge. [Silk90]

Also AR coating. A layer of material of lower
refractive index of just the right thickness (1/4 wave) is deposited on
the optical surface to be coated. More complex coatings are possible
which cover a large wavelength range. [McL97]

Apastron

The point in the orbit of one component of a
binary system where
it is farthest from the other. [H76]

Aperture

(a) The effective diameter of the primary mirror or
lens of a telescope. [H76] (b) A measure of the effective diameter (d) of a
mirror or lens compared with its focal distance (f):

aperture = d/f

Thus a 50-mm camera lens may be used with an aperture diameter of 12.5
mm. Then, aperture = 12.5/50. This is usually described with the
f-number. In this case the aperture diameter is f/4,
often written as f4.
The transmitted light intensity depends on aperture diameter, so that
I is proportional to d2. However, large
apertures lead to large aberrations although diffraction effects are
more serious at small apertures. In many optical instruments, iris
diaphragms vary the aperture to obtain the optimum results. [DC99]

Aperture Efficiency

The ratio of the effective aperture of the
antenna, A, to
its geometric aperture, Ag = d2 / 4. The beam and
aperture efficiencies are related by A = B2 / AgM, where M is the
solid angle of the main beam. [H76]

Aperture Function

In radio astronomy, a distribution of direction
assignments
applying to a uniform background. [H76]

Aperture Photometry

Usually refers to magnitude measurements made from
digital images by deriving the flux that would have been recorded within
a circular aperture large enough to enclose the star's seeing disk. [McL97]

The method of combining the signals received by
several smaller telescopes distributed over a very large area or
baseline to provide the angular resolution of a much large telescope.
Used extensively in radio astronomy, e.g. the VLA. [McL97]

Apex

see Solar Apex

Aphelion

The point in a planetary
orbit that is at the greatest
distance from the Sun. [S92]

Aplanatic Lens

A lens designed so as to minimize both its
astigmatic and coma aberration. [DC99]

Aplanatic System

A system of three lenses which, taken together,
correct for
spherical aberration, chromatic aberration, and coma. [H76]

Apocenter

The point in the orbit of one component of a
binary system which
is farthest from the center of mass of the system. [H76]

Apochromatic Lens

A lens designed to correct for chromatic
aberration at three different wavelengths. Apochromatic lenses are
constructed of three or more kinds of glass. They thus have better
correction than achromatic lenses, which correct at two different
wavelengths (usually in the red and blue regions of the
spectrum). see Achromatic Lens[DC99]

Apodization

A mathematical process performed on the data
received from an
interferometer before carrying out the calculations of the Fourier
transformation to obtain the spectrum, in order to modify the
instrumental response function. [H76]

Apogee

The point at which a body in
orbit around the Earth reaches its
farthest distance from the Earth. [S92]

Apollo Asteroid

One of a small group of asteroids whose orbits
intersect that of
Earth. They are named for the prototype, Apollo (P =
622d, a = 1.486 AU, e = 0.57, i =
6°.4). [H76]

Apollo Space Program

Successful US lunar exploration program in
which the Apollo spacecraft 1 to 6 were unmanned;
7 to 10 were manned but did not land; and 11,
12 and 14 to 17 landed and returned safely. (Apollo 13 was an
aborted
mission.) The first men to land on the Moon were Neil Armstrong and
Edwin Aldrin, from Apollo 11, on 20 July 1969. The final
Apollo flight (17) lasted from 7 to 19 December 1972, and left
a considerable quantity of exploratory devices on the lunar surface. [A84]

Because radiation travels at different speeds in
different media, the apparent depth or thickness of a transparent
sample is not the same as its real depth or thickness. The effect is
very obvious when one looks down into a glass of water or a clear
pool. It is associated with the fact that a long object partly
submerged in water seems bent at the water surface.
The refractive constant of the substance can be measured on this basis:

refractive index = real depth / apparent depth

The relation is used in a number of methods for finding the refractive
constant of a transparent medium. It applies to all wave radiations,
not just to visible radiation. [DC99]

Apparent Magnitude

(a) A measure of how bright a star looks in the
sky. The brighter the
star, the smaller the apparent magnitude. A star that is one magnitude
brighter than another (e.g., +1 versus +2) looks 2.5
times brighter. The brightest star of all, of course, is the Sun,
whose apparent magnitude is -26.74, followed by Sirius, whose apparent
magnitude is -1.46, Canopus (-0.72), Alpha Centauri (-0.27), Arcturus
(-0.04), and Vega (+0.03). Stars of the Big Dipper are fainter, most
of them around magnitude +2. On a clear, dark night, the unaided eye
can see stars as faint as apparent magnitude +6, and the largest
telescopes penetrate to apparent magnitude +30. [C95](b) Measure of the observed brightness of a celestial
object as seen
from the Earth. It is a function of the star's intrinsic brightness,
its distance from the observer, and the amount of absorption by
interstellar matter between the star and the observer. The
mv, of Sun, -26.5 mag. A sixth-magnitude star is just
barely visible to the naked eye. [H76]

Apparent Place

The position on a
celestial sphere, centered at
the Earth, determined by removing from the directly observed position of a
celestial body the effects that depend on the
topocentric location of
the observer; i.e., refraction,
diurnal aberration. see
Aberration, Diurnal), and geocentric (diurnal)
parallax. Thus the position
at which the object would actually be seen
from the center of the Earth, displaced by planetary aberration
(except the diurnal part - see
Aberration, Planetary; Aberration, Diurnal) and referred to the
true equator and equinox. [S92]

Apparent Solar Day

Interval between two successive culminations of
the Sun - i.e.,
the period from apparent noon to apparent noon. The apparent Solar day
is longest in late December. [H76]

Apparent Solar Time

The measure of time based on the diurnal motion
of the true
Sun. The rate of diurnal motion undergoes seasonal variation because
of the obliquity of the
ecliptic and because of the
eccentricity of
the Earth's orbit. Additional
small variations result from
irregularities in the rotation of the Earth on its axis. [S92]

( Boo) (a) A beautiful orange star that is the
brightest in the
constellation Bootes and the fourth brightest in the night sky. It
lies 34 light-years away and is a member of the thick-disk
population. Historically, Arcturus is famous because it was one of the
first stars to have its proper motion measured. [C95](b) An old subgiant disk star (K2 IIIp,
mv = 0.06) about 11 pc distant. [H76]

Areas, Law of

see Kepler's Second Law

Argand Diagram

a diagram in which the length and phase-angle of a
complex quantity is displayed. {D89]

Argelander Method

A method of classifying stars according to image
size. If the
sequence stars are labeled a, b, etc., in order of image
size and if the image size of a variable appears to be, say, 0.7 of
the way from sequence star a to sequence star b, its
brightness is listed as a7b. (also called the step method
)[H76]

Argon

An inert colorless odorless monatomic element of the
rare-gas group. It forms 0.93% by volume of air. Argon is used to
provide an inert atmosphere in electric and fluorescent lights, in
welding, and in extracting titanium and silicon. The element forms no
known compounds.
Symbol: Ar; m.p. -189.37°C; b.p. -185.86°C; d. 1.784 kg
m-3 (0°C); p.n. 18; r.a.m. 39.95. [DC99]

Argument of the Perihelion ()

Angular distance (measured in the plane of the
object's orbit and
in the direction of its motion) from the ascending node to the
perihelion point. [H76]

Ariel

Second satellite of Uranus about 1600 km in
diameter, discovered
by Lassell in 1851. Period 2.52 days. [H76]

Aristotelian Physics

Physics as promulgated by Aristotle; includes
the hypothesis that
our world is comprised of four elements, and that the Universe beyond
the moon is made of a fifth element and so is fundamentally different
from the mundane realm. [F88]

Arm Population

Young stars typical of those found in spiral arms
(Population I
stars). [H76]

Armillary Sphere

Ancient Greek, Arabic and medieval alt-azimuth
device, comprising a calibrated ring fixed in the meridian plane,
within which a second concentric ring, also calibrated, was mobile
around a vertical axis. [A84]

Aromatic, Aromaticity

In organic chemistry, carbon which has bonds that
are between single
and double bonds (e.g., the molecule has a bond with a delocalized
electron.) [SEF01]

Array

In radio astronomy, an arrangement of antenna
elements designed to
produce a particular antenna pattern. [H76]

Arrival Time

see Dispersion

Arrow of Time

The direction, apparently inviolable, of the
"flow" of time that distinguishes the past from the future.The
direction, apparently inviolable, of the "flow" of
time that distinguishes the past from the future. [HH98]

Arsenic

A toxic metalloid element existing in several
allotropic forms; the most stable is a brittle gray metal. It is used
in semiconductor devices, alloys, and gun shot.
Symbol: As; m.p. 817°C (gray) at 3 MPa pressure; sublimes at
616°C (gray); r.d. 5.78 (gray at 20°C); p.n. 33;
r.a.m. 74.92159. [DC99]

In the orbit of a Solar-System body, the point
where the body
crosses the ecliptic from south to north: for a star, out of the plane
of the sky toward the observer. [H76]

Ashen Light

A faint glow from the unlit side of Venus when it
is in the
crescent phase. Its cause is unknown; it may be the Venusian analog to
terrestrial airglow. [H76]

Asinh Magnitude

Magnitudes
expressed as the inverse hyperbolic sine (or ``asinh''), sometimes
referred to informally as luptitudes. The transformation from
linear flux measurements to asinh magnitudes is designed to be virtually
identical to the standard astronomical magnitude at high
signal-to-noise ratio, but switches over to linear behavior at low S/N
thereby accommodating even negative values of flux, where the
logarithm, as used in the Pogson magnitude, fails. [sinh-1(x) =
asinh(x)*ln(x+SQRT(x2+1))]

ASIC

Application Specific Integrated Circuit

Aspect

The apparent position of any of the planets or
the Moon relative
to the Sun, as seen from Earth. [S92]

Aspect Ratio

Ratio of the major axis (e.g., of a rocket) to
the minor axis. (Of
a fusion device) ratio of the plasma diameter to the major diameter of
the torus. [H76]

Aspheric

An optical surface with departures in shape from a
perfect
sphere in order to cancel optical imperfections or aberrations. [McL97]

Association

A sparsely populated grouping (mass range
102-103 M) of very young, massive stars lying along a spiral
arm of the Milky Way, whose spectral types or motions in the sky indicate
a common origin. The star density is insufficient for gravitation to
hold the group together against shear by differential galactic
rotation, but the stars have not yet had time to disperse
completely. OB associations are composed of stars of spectral types
O-B2; T associations have many young T Tauri stars. The
internationally approved designation for associations is the name of
the constellation followed by an arabic numeral - e.g., Perseus
OB2. [H76]

(a) A small rocky body that orbits a star. In the
Solar System,
most asteroids lie between the orbits of Mars and Jupiter. The largest
asteroid is Ceres, about 900 kilometers in diameter. [C95](b) A small planet-like body of the Solar System,
<e> ~ 0.15,
<i> ~ 9°.7. More than 1800 have been catalogued, and
probably millions of smaller ones exist, but their total mass would
probably be less than 3 percent that of the Moon. Their densities are
poorly known (about 2.6 g cm-3), but they suggest a
composition similar to carbonaceous chondrite. The bright asteroids
are presumably original condensations and those fainter than about
14-15 mag are collision fragments. Asteroids and short-period comets
have some orbital similarities. Also called minor planet [H76]
(c) Also called planetoids or minor planets, the
asteroids are
tiny planets most of which orbit the Sun between Mars and Jupiter.
The largest - and the first discovered - is Ceres, with a diameter of
1,003 km. It is estimated that there may altogether be no fewer than
40,000. A few have very elliptical orbits and cross the orbits of
several other (major) planets. One or two even have their own
satellites (moons). [A84]

Asteroid Belt

A region of space lying between Mars (1.5 AU) and
Jupiter (5.2
AU), where the great majority of the asteroids are found. None of the
belt asteroids have retrograde motion. [H76]

Astigmatism

(a) A common eye defect in which the
observer cannot
focus clearly on objects at any distance. The cause is usually a
non-spherical cornea. Visual astigmatism may be corrected with a lens
with a suitable degree of cylindrical curvature. [DC99] (b) An optical aberration in which off-axis points tend to
become elongated ellipses. [McL97] (c) Affects rays neither close nor parallel to the
axis. The cone of rays through a lens from an off-axis object does not
focus at a point. Instead, two images in the form of short lines are
formed at different distances from the lens. Between the two the image
appears blurred. Mirrors forming images of off-axis points show a
similar defect. The best method of minimizing astigmatism is to reduce
the aperture with stops, thus allowing light only through the center
of the lens.

(a) Ancient Arabic and medieval alt-azimuth device
comprising two or more
flat, metal, calibrated discs, attached so both or all could rotate
independently. For early navigators and astronomers it acted as
star-chart, compass, clock and calendar. [A84](b) Sighting instrument employed since antiquity to
determine the
elevation above the horizon of celestial objects. Eventually replaced
by the sextant. [F88]

Astrology

(a) The belief that human affairs and people's
personalities and
characters are influenced by (or encoded in) the positions of the
planets. [F88](b) Divination using the positions of the planets,
the Sun and
the Moon as seen against the stars in the constellations of the zodiac
- a "science" almost as old as homo sapiens. Although at one stage
in history astrology and astronomy were almost synonymous- the latter
has advanced so far during the last three centuries that the two now
bear little relation to each other. [A84]

Astrometric Binaries

see Binary System

Astrometric Ephemeris

An ephemeris of a
Solar-System body in which the tabulated
positions are essentially comparable to catalog
mean places of stars
at a standard epoch. An
astrometric position is obtained by adding to
the geometric position, computed
from gravitational theory, the
correction for light-time. Prior
to 1984, the E-terms of annual
aberration (see Aberration, Annual;
Aberration, E-terms of) were also added to the
geometric position. [S92]

Astrometry

The branch of astronomy that deals with
measuring the
positions of celestial objects, especially stars. Astrometrists
measure parallaxes and proper motions, which allow astronomers to
determine the distances and velocities of the stars. [C95]

Astronomical Color Index

Difference in a star's brightness when
measured on two selected wavelengths, in order to determine the star's
temperature. Cooler stars emit more light at longer wavelengths (and
so appear redder than hot stars). Modern methods involve photoelectric
filtering and the UBV system. [A84]

Astronomical Coordinates

The longitude and latitude of a point on the
Earth relative to the
geoid. These coordinates are
influenced by local gravity
anomalies. see Zenith;
Longitude, Terrestrial;
Latitude, Terrestrial) [S92]

Astronomical Twilight

The period from sunset to the time that the Sun
is 18° below
the horizon; or the corresponding period before sunrise. [H76]

Astronomical Unit

(a) Mean distance between the Earth and the Sun:
149,598,500km. [A84](b) The AU is the preferred unit for
distances within the Solar System. Mercury, the innermost planet,
lies on average 0.39 AU from the Sun; Pluto, normally the
farthest planet, lies on average 39.5 AU from the Sun. [C95](c) The mean distance between the Earth and the
Sun. The astronomical
unit is defined as the length of the radius of the unperturbed
circular orbit of a body of negligible mass moving around the Sun with
a sidereal angular velocity of 0.017202098950 radian per day of 86,400
ephemeris seconds. AU = 1.496 × 1013 cm 500 lt-sec. [H76](d) The mean distance from the earth to the sun,
equal to 92.81
million miles or 499.012 light-seconds. [F88](e) The radius of a circular
orbit in which a body of negligible
mass, and free of perturbations,
would revolve around the Sun in 2 / k days, where k is the
Gaussian gravitational constant.
This is slightly less than the
semi-major axis of the Earth's
orbit. [S92]

(a) The science that studies the physics and
chemistry of
extraterrestrial objects. The alliance of physics and astronomy, which
began with the advent of
spectroscopy, made it possible
to investigate what celestial objects are and not just
where they are. [F88] (b) All applications of the laws of physics, chemistry and
the other physical sciences to the understanding of astronomical
phenomena. [D89]

Asymmetric Drift

The negative of the mean V velocity of a stellar
population. In general, the older the stellar population, the more
negative the V velocity and therefore the greater the asymmetric
drift. The young thin disk has an asymmetric drift of 0 kilometers per
second, whereas the halo has an asymmetric drift of 200 kilometers per
second. [C95]

Globular cluster stars, which are found in that
part of the HR diagram that connects the top pf the giant tip with the
horizontal branch. [JJ95]

Asymptotic Freedom

(a) A term used to describe the observed
decrease in the intrinsic
strength of the color force between quarks as they are brought closer
together. At asymptotically small separations, the quarks are
virtually free. This is in contrast to the electromagnetic force whose
intrinsic strength increases as two charged particles approach each
other. [CD99] (b) A force of interaction between particles is said
to be asymptotically free if it becomes weaker as the energy of the
interacting particles increases. Empirically the force between-quarks
in a proton or neutron is found to be asymptotically free, a feature
that can be explained by assuming that the force is described by a
Yang-Mills theory. [G97] (c) In general, there are strong, QCD forces between
quarks due to the exchange of gluons. Asymptotic freedom is the
principle which says that these forces become weaker for very close
encounters between quarks, so that the quarks become `free' of the
forces at very short distances. In high-energy collisions, the quarks
do sometimes become very close and, since they then behave almost like
free particles, it is not too difficult to calculate their motion
during the close collision. [D89]

ATB

Acronym for After The Bang; usually used in
reference to time elapsed since the big bang. [G99]

Atmosphere

(a) Mantle of gases round a star planet or moon,
sometimes even
forming the apparent surface of the body. For a body to retain an
atmosphere depends on the body's gravity, and the temperature and
composition of the gases. The atmosphere of the Earth is, by volume,
78% nitrogen and 21% oxygen (with 1% of other gases); mean
atmospheric pressure at the surface is 10,330 kg/m2, and is also
referred to as atmosphere. [A84](b) Unit of pressure. 1 atm = 1.013 bars. [H76] (b) A unit of pressure equal to 14.07 lb
in2, 101 325 pascals or 760 torr. [JM92]

Atmosphere (Solar)

The gaseous outer layers of the Sun, including,
from the deeper
layers outward, the photosphere, the chromosphere, and the corona. The
atmosphere constitutes those layers of the Sun that can be observed
directly. [H76]

Atmospheric Dispersion Corrector

An optical device usually comprising
two thin prisms which can rotate to compensate for the elongation of a
star image caused by the wavelength dependence of the refractive index
of air. [McL97]

Atmospheric Extinction

Decrease in the intensity of light from a
celestial body due to
absorption and scattering by the Earth's atmosphere. The extinction
increases from the zenith to the horizon and affects short wavelengths
more than long wavelengths, so that objects near the horizon appear
redder than they are at the zenith. [H76]

Atom

(a) The smallest component of matter that retains its
chemical
properties. An atom consists of a nucleus and at least one electron. [HH98](b) The building block of matter. The nucleus of an
atom consists of
one or more protons and may contain neutrons as well; any electrons
surround the nucleus. The number of protons in the atom - the atomic
number - determines the element. [C95] (c) The smallest part of an element that can take part
in a
chemical reaction. Atoms consist of a small dense positively charged
nucleus, made up of neutrons and protons, with electrons in a cloud
around this nucleus. The chemical reactions of an element are
determined by the number of electrons (which is equal to the number of
protons in the nucleus). All atoms of a given element have the same
number of protons (the proton number). A given element may have two or
more isotopes, which differ in the number of neutrons in the nucleus.
The electrons surrounding the nucleus are grouped into shells -
i.e. main orbits
around the nucleus. Within these main orbits there may be
sub-shells. These correspond
to atomic orbitals. An electron in an atom is specified by four quantum
numbers:
1. The principal quantum number (n) can have values 1, 2,
etc. The corresponding shells are denoted by letters K, L, M, etc.,
the K shell (n = 1) being the nearest to the nucleus. The
maximum number of electrons in a given shell is
2n2. This quantum number has the largest effect on
the energies of the states; high values of n correspond to
weakly bound (higher energy) electrons.
2. The orbital quantum number (l), which specifies the angular
momentum. For a given value of n, l can have possible values of
n-1, n-2,... 2, 1, 0. For instance, the M shell
(n = 3) has three sub-shells with different values of l
(0, 1, and 2). Sub-shells with angular momentum, 0, 1, 2, and 3 are
designated by letters s, p, d, and f. This quantum number has the
second largest effect on the energies; higher values of l give
moderately higher energy electrons.
3. The magnetic quantum number (m). This can have values -l,
-(l - 1) ... 0 ... +(l + 1), + l. It determines the
orientation of the electron orbital in a magnetic field. States with
the same values of n and l but different values of
m have the same energy in the absence of a magnetic field, but
differ slightly when a field is applied.
4. The spin quantum number (ms), which specifies the
intrinsic angular momentum of the electron. It can have values +½
and -½. Quantum states in which the spin is parallel to the
orbital angular momentum are at slightly higher energy than ones in
which it is antiparallel. This results, for example, in the fact that
the yellow light from a sodium lamp has two very close lines in its
spectrum.
Each electron in the atom has four quantum numbers and, according to
the Pauli exclusion principle, no two electrons can have the same set
of quantum numbers. This explains the electronic structure of atoms. [DC99] (c) An atom has a nucleus surrounded by electrons bound together
by the
electromagnetic force. Ninety-two different atoms occur naturally, making
ninety-two different chemical elements, with nuclei having one to
ninety-two protons. The atoms are electrically neutral. The diameter of
an atom is about 10,000 times larger than the diameter of its
nucleus. [K98]

Atomic Hydrogen

Individual hydrogen atoms that do not belong
to molecules.
In its neutral form (HI), atomic hydrogen consists of a proton and
an electron and generates radio waves that are 21 centimeters
long. In its ionized form (HII), atomic hydrogen is simply a
proton. HII regions look red because a few of the protons capture
electrons, which can radiate red light as they settle into position
around the protons. [C95]

Atomic Mass Number

The total number of protons and neutrons in
an atom's
nucleus. For example, oxygen-16 has a mass number of sixteen,
because it has eight protons and eight neutrons. [C95]

Atomic Mass Unit

(a) A unit of mass used for atoms
and molecules, equal to 1/12 of the mass of an atom of carbon-12. It
is equal to 1.660 33 × 10-27 kg. [DC99] (b) In the notation of physics, adopted by
international agreement in
1961, one-twelfth the mean mass of an atom of 126 (including the
orbital electrons). Because of the mass defect, the amu is
slightly less than the mass of a hydrogen atom, so one H atom has
1.007825 amu. 1 amu = 1.66 × 10-24 g. The energy equivalent
of 1 amu is 931 MeV. [H76] (c) The masses of atoms and molecules are generally
given in atomic mass units.
These units are based on a scale in which the mass of the carbon isotope
126C is taken to be 12. This makes one atomic mass
unit equivalent to
1.6605402 × 10-27 kg. The unit is sometimes called a
dalton. Atomic masses
were originally given as atomic weights on a scale where the mass of the
hydrogen atom was unity. In 1885 Ostwald suggested if atomic weights were
expressed on a scale in which the mass of oxygen was 16, more of the
elements
would have integral numbers for their atomic weights. The discovery of
oxygen isotopes led to the adoption of two atomic weight
scales[27]. The one
used by chemists was based on the figure 16 representing the average
mass of
the oxygen atom in its natural state
(16O:18O:17O in the abundance
506:1:0.204), whereas that used by the physicists considered the oxygen
isotope 16O as the basic unit on their scale. The ratio of
atomic mass on the
physical scale to atomic mass on the oxygen chemical scale[28] was
1.000272±0.00005.

In 1960[29] the International Union of Pure and Applied Physics followed
the proposal made a year earlier by the International Union of Pure and
Applied Chemistry that all atomic weights should be based on the
126C scale.
This enabled more isotopes to have integral mass numbers than would have
been possible on the oxygen scale. This reduced the values given on the
previous chemical scale by a factor of 1.000043 and changed the values
of the
Faraday and Avogadro's constant by a similar amount. Values on the physical
scale can be converted to the new scale by multiplying by 0.999685. [JM92]

Atomic Number

(a) The number of protons in an atom's nucleus.
This determines
the type of element. For example, hydrogen has an atomic number of
one, so all hydrogen atoms have one proton; helium has an atomic
number of two, so all helium atoms have two protons; and oxygen has an
atomic number of eight, so all oxygen atoms have eight protons. [C95] (b) The atomic number was introduced by
J. A. R. Newlands[30] (1837-1898) in
1865 to describe the position of an element in the periodic table. The
work of
Rutherford and Moseley (1913) showed that the atomic number also indicated
both the number of electrons in an atom and the number of positive charges
in the nucleus[31]. The values of the atomic numbers of the elements so far
discovered lie between one for hydrogen and 103 for lawrencium. [JM92]

Atomic Orbital

Representation of the electron cloud surrounding
an atom. Named by
primary quantum number and shape (i.e., 1s, 2s, 2p). [SEF01]

The theory that matter is made up of atoms that
combine to form molecules. Each chemical element has a particular type
of atom, which may join with like atoms to form molecules of the
element, or with atoms of other elements to form molecules of a
compound. The atom consists of a dense positively charged nucleus
containing protons and neutrons, surrounded by electrons. The number
of protons in the nucleus determines the number and distribution of
the electrons, which are held by the positive charge of the
nucleus. Because the outer electrons form the chemical bonds between
atoms, the chemical properties of an element depend on the electronic
structure of the atom, and therefore also on the number of
protons. The number of neutrons in the nucleus may vary, forming
different isotopes of an element. These cannot usually be separated by
chemical means. [DC99]

Atomic Time

Time based on the atomic second (see
Second). Atomic time was
officially adopted 1972 January 1. From 1972 January to 1974 January
1, 3 leap seconds had to be introduced to keep atomic time within 0.7
seconds of Universal Time. [H76]

Atomic Weight

The mean atomic mass of a particular element in
atomic mass
units. [H76]

Atoms

The fundamental units of a chemical element. An
atom consists of a nucleus, which may contain
protons and neutrons, and electrons, which occupy shells that
surround the nucleus and are centered on it. [F88]

Attenuation

(a) The reduction of intensity of a
radiation as it
passes through a medium. It includes reductions due to both absorption
and scattering.
(b) Reduction in current, voltage, or power of
an electrical signal passing through a circuit. [DC99] (c) The falling off of the energy density of
radiation with distance
from the source, or with passage through an absorbing or scattering
medium. [H76]

Attenuation Factor

In a rocket-borne or satellite-borne telescope,
the ratio of the
expected counting rate to the observed counting rate. [H76]

Attitude

Position of a rocket with respect to the horizon
or some other
fixed reference plane. [H76]

Attractor

(a) A mechanical system may be such that its dynamical
evolution causes it to approach a stable end-state. In the phase space
representing the system, the representative point tends to a fixed set
of points called an attractor. The attractor may be a point, a line,
or a fractal. [D89] (b) The point or set of points in phase space to which a
changing system moves with time. The idea of an attractor for a system
comes from chaos theory. The attractor of a system may be a single
point (in which case the system reaches a fixed state that is
independent of time). Alternatively, it may be a closed curve, known
as a limit cycle. This is the type of behavior found in
oscillating systems. In some systems, the attractor is a curve that is
not closed and does not repeat itself. This, known as a strange
attractor, is characteristic of chaotic systems. see also
Chaos Theory; Phase Space[DC99]

Having a stellar spectral type of A, that is, hot and
white, like
Sirius and Vega. [C95]

AU

Astronomical Unit

Auger Effect

(a) The ejection of an electron from an atom or ion
without the emission of radiation (x-rays or gamma rays). It results
from the de-excitation of an excited electron within the atom. It can
be regarded as the internal conversion of the photon that would
otherwise have been emitted. [DC99] (b) A radiationless quantum jump that occurs in the
X-ray region. When
a K-electron is removed from an atom and an L-electron drops into the
vacancy in the K-shell, the energy released in the latter transition
goes not into radiation, but into the liberation of one of the
remaining L-electrons. [H76]

Augmentation

The amount by which the apparent
semidiameter of a celestial
body, as observed from the surface of the Earth, is greater than the
semidiameter that would be observed from the center of the Earth. [S92]

An eclipsing binary with an invisible supergiant
companion. The
primary is an extremely luminous A8 Ia supergiant of 30 M in a
post-main-sequence stage of evolution; the secondary may be a
collapsed star or black hole. The period of the system is about 27
years. Probably on the order of 1 kpc distant. It has at least six
components. [H76]

In general, binaries with a K supergiant primary and a
main-sequence secondary. [H76]

Aurora

(a) Spectacular array of light in the night sky,
caused by charged
particles from the Sun hitting the Earth's upper atmosphere. The
aurora borealis is seen in the north of the Northern hemisphere; the
aurora australis in the south of the Southern. [A84](b) Light radiated by ions in the Earth's atmosphere,
mainly near the
geomagnetic poles, stimulated by bombardment by energetic particles
ejected from the Sun (see Solar Wind). Aurorae appear about 2 days
after a Solar flare and reach their peak about 2 years after sunspot
maximum. [H76]

Auto-Catalysis

the ability of certain chemicals to enhance by their
presence the rate of their own production in a sequence of chemical
reactions. Part of the more general class of feedback processes. [D89]

Auto-Ionization

(a) The spontaneous ionization of excited atoms,
ions, or molecules, as in the Auger effect. see Auger Effect;
Ionization[DC99] (b) A phenomenon occurring when a discrete
double-excitation state of
an atom lies in the ground-state continuum. In the autoionization
process one of the excited electrons is ejected, leaving the ion in an
excited state (see Dielectronic Recombination; see also Auger
Effect). (also called Pre-ionization)[H76]

(a) Symbol: NA number of
particles in one mole of a substance. Its value is 6.002 52 ×
1023. [DC99] (b) The Avogadro constant is the number of atoms in a
gram-atom (mass in grams
numerically equal to the atomic weight) or the number of molecules in a
gram-molecule (mass in grams numerically equal to the molecular weight). It
is named after Amadeo Avogadro (1776-1856), an Italian who in 1811
introduced the famous hypothesis. The value is 602.213 67(36) ×
1021 mol-1[JM92]

Avogadro's Law

Equal volumes of all gases at the same
temperature and pressure contain equal numbers of molecules. It is
often called Avogadro's hypothesis. It is strictly true only
for ideal gases. [DC99]

Avogadro's Number

(6.02 × 1023): The number of
atoms in 12 grams of 12C; by extension,
the number of atoms in a gram-atom (or the number of molecules in a
mole) of any substance. [H76]

Avoidance

The fact that galaxies appear to
"avoid" the Milky Way, and are most numerous in other parts of the
sky. When galaxies were known as spiral
nebulae and their nature
was not yet understood, avoidance was thought by some researchers to
indicate a connection between them and the Milky Way. Now the effect
is understood to be due to dark clouds of dust and gas in our galaxy,
which obscure our view of the Universe beyond in those quarters of the
sky. [F88]

AXAF

Advanced X-ray Astronomical Facility

Axion

A hypothetical spin-0 particle with a very small
mass of 10-5-10-3 eV.
It was postulated in order to provide a natural solution to the
"strong CP problem".

Axis

Theoretical straight line through a celestial
body, around which
it rotates. [A84]

Axisymmetric Collapse

Collapse of mass in such
a way that the mass maintains the symmetry of a cylinder. [LB90]

Azeotropic Mixture

Aseotrope: A mixture of two liquids that
boils without any change in composition. The proportions of components
in the vapor are the same as in the liquid. Azeotropic mixtures cannot
be separated by distillation. [DC99]

Azimuth

(a) Directional bearing around the horizon, measured
in degrees from north (0°). [A84](b) Angular distance from the north point eastward to
the intersection
of the celestial horizon with the vertical circle passing through the
object and the zenith. [H76]

Azimuthal Quantum Number (k)

A measure of the minor axis of an elliptic
orbital of an electron
according to the Bohr-Sommerfeld theory. [H76]